/**
 * @author alteredq / http://alteredqualia.com/
 *
 * Convolution shader
 * ported from o3d sample to WebGL / GLSL
 * http://o3d.googlecode.com/svn/trunk/samples/convolution.html
 */

THREE.ConvolutionShader = {

  defines: {

    "KERNEL_SIZE_FLOAT": "25.0",
    "KERNEL_SIZE_INT": "25"

  },

  uniforms: {

    "tDiffuse":        { value: null },
    "uImageIncrement": { value: new THREE.Vector2( 0.001953125, 0.0 ) },
    "cKernel":         { value: [] }

  },

  vertexShader: [

    "uniform vec2 uImageIncrement;",

    "varying vec2 vUv;",

    "void main() {",

      "vUv = uv - ( ( KERNEL_SIZE_FLOAT - 1.0 ) / 2.0 ) * uImageIncrement;",
      "gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

    "}"

  ].join( "\n" ),

  fragmentShader: [

    "uniform float cKernel[ KERNEL_SIZE_INT ];",

    "uniform sampler2D tDiffuse;",
    "uniform vec2 uImageIncrement;",

    "varying vec2 vUv;",

    "void main() {",

      "vec2 imageCoord = vUv;",
      "vec4 sum = vec4( 0.0, 0.0, 0.0, 0.0 );",

      "for( int i = 0; i < KERNEL_SIZE_INT; i ++ ) {",

        "sum += texture2D( tDiffuse, imageCoord ) * cKernel[ i ];",
        "imageCoord += uImageIncrement;",

      "}",

      "gl_FragColor = sum;",

    "}"


  ].join( "\n" ),

  buildKernel: function ( sigma ) {

    // We lop off the sqrt(2 * pi) * sigma term, since we're going to normalize anyway.

    function gauss( x, sigma ) {

      return Math.exp( - ( x * x ) / ( 2.0 * sigma * sigma ) );

    }

    var i, values, sum, halfWidth, kMaxKernelSize = 25, kernelSize = 2 * Math.ceil( sigma * 3.0 ) + 1;

    if ( kernelSize > kMaxKernelSize ) kernelSize = kMaxKernelSize;
    halfWidth = ( kernelSize - 1 ) * 0.5;

    values = new Array( kernelSize );
    sum = 0.0;
    for ( i = 0; i < kernelSize; ++ i ) {

      values[ i ] = gauss( i - halfWidth, sigma );
      sum += values[ i ];

    }

    // normalize the kernel

    for ( i = 0; i < kernelSize; ++ i ) values[ i ] /= sum;

    return values;

  }

};